IMac – Mojave screen saver starts randomly

If your computer is locked, the screensaver settings for the root user account is used instead of your own. You can change those settings according to the answer to this question: How can I set the Mac OS X screensaver when not logged in?

I noticed the same effect in full Yosemite installation: screensaver starts when the computer (macbook air) is put down to sleep. If we add to this the battery "eating" behaviour I observed as well. I have unchecked every item that can use power, icloud and handsoff included, but the system itself needs a lot more energy than Mavericks. The general picture of power management seems to be worse than ever.